Generally, when pixels are placed in a rectangular raster, image boundaries extending obliquely, i.e., not in a major direction of the raster, are reproduced as a series of steps. This effect, known as "staircasing", has a negative influence on image quality and may rapidly lead to unacceptable results in the event of indiscriminate enlargement of the pixel image, e.g., by doubling the number of pixels in both major directions. Prior to any enlargement or, generally, to any change of resolution of a pixel image, it is recommended that the original non-rastered image be reconstructed, as far as possible, by converting the pixel image into a non-rastered form or at least into a raster pattern having a very high resolution with respect to the resolution of the original pixel pattern. By such reconstruction the staircasing disappears. The reconstructed image is then mapped on a raster with the required resolution.
One such method is described in U.S. Pat. No. 4,242,678 where the value of the pixels is determined for each pixel in a pixel window, a 3.times.3 pixel window, with the required pixel as the centerpoint. This combination of pixel values is searched in a previously made table containing a replacement surface for each possible combination. This replacement surface, which has a form rastered with very high resolution, is put in the place of the required pixel and converted into a pattern of new pixels having the required different resolution. By performing this replacement for all the original pixels, a new pixel pattern is constructed with the required resolution.
A disadvantage of the known method is that it is limited to small analysis are as (3.times.3 pixels) and even smaller processing zones (1 pixel) so that larger structures cannot be adequately treated. As a result, oblique image boundaries, for example, extending at an angle other than 45.degree., are still reproduced with some degree of staircasing in replacement surfaces. Enlargement of the analysis window admittedly provides an improvement but results in a gigantic enlargement of the comparison table and hence processing time.
Accordingly, it is an object of the present invention to provide a method and an apparatus for generating correct replacement surfaces for image boundaries extending at any angle.